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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Sending Variable Length Messages with GNAT.Sockets
Date: Mon, 09 May 2005 20:16:18 +0100
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Date: 2005-05-09T20:16:18+01:00
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"markp" <markwork66@yahoo.com> writes:

> I still have a fundamental problem. I am working on legacy code in
> which data is defined in records (or arrays of records) which are sent
> in variable lengths. We use address, byte count to send the exact
> number of bytes. Using Streams, it seems they type has to be of
> Stream_element_Array. The only way I can see to send/receive these
> "variable" messages is to first read our message header which contains
> the number of bytes in the message. Then, create a Stream_Element_Array
> of the exact size for the next read. Then, do an unchecked conversion
> or a copy bytes routine to get the data into my data type for
> processing. Does this sound correct?

Sounds pretty good to me; though the compiler is going to complain
about the unchecked conversion if your data type isn't the size
specified in the header?

Assuming you have type my_array_type is array (Natural range <>) of
your record type, try something like

   read the number of bytes;
   compute the number of array elements;
   handle the error if it doesn't match exactly;
   declare
      Readable : my_array_type (1 .. number_of_array_elements);   
   begin
      my_array_type'Read (str, Readable);
      return Readable;  --  or something related
   end;

> Lastly, using the 'Read function on streams, am I guaranteed to block
> until I receive all bytes?

I believe so (unless the other end closes, or you have a non-blocking
socket, in which case I'd expect End_Error).